In recent years, there has been a demand for pneumatic tires to have a small rolling resistance, excellent fuel economy performance, and excellent grip performance. In order to satisfy these demands, there is a known technique of compounding a reinforcing filler such as a styrene-butadiene copolymer or silica with a rubber composition constituting a cap tread of a tire. In order to further enhance the wear resistance or rubber hardness of the rubber composition, it has been proposed to compound polybutadiene or a silica having high reactivity, for example, but in this case, there has been a problem in that the rubber strength or rubber hardness decreases or the processability is diminished.
Japanese Unexamined Patent Application Publication No. 03-239737 describes that a pneumatic tire using a rubber composition prepared by compounding a silica and a styrene-butadiene copolymer having a specific arrangement of styrene-derived units for a tread simultaneously achieves wet skid resistance, rolling resistance, and wear resistance. However, this rubber composition has not been capable of sufficiently satisfying the demands of consumers because the rubber hardness cannot necessarily be sufficiently enhanced.
Japanese Unexamined Patent Application Publication No. 57-179212 describes a styrene-butadiene copolymer in which, a long-chain styrene block content is not greater than 5 wt. %, a simple chain content having one styrene-derived unit is not less than 50 wt. % relative to the total styrene content in the styrene-butadiene copolymer, and a total styrene content is from 10 to 30 wt. % of the styrene-butadiene copolymer content. However, the styrene-butadiene copolymer of Japanese Unexamined Patent Application Publication No. 57-179212 was not sufficient to enhance the rubber hardness of the rubber composition.